Nils, Discipline Enforcer
Legendary Creature — Human Cleric
At the beginning of your end step, for each player, put a +1/+1 counter on up to one target creature that player controls.
Each creature with one or more counters on it can't attack you or planeswalkers you control unless its controller pays 
, where X is the number of counters on that creature.
Magali Villeneuve
Commander 2021 (C21)
• #20Rulings
For each player, choosing a target creature that player controls is optional.
(wotc • 2021-04-16)
Your opponents can choose not to pay to attack with a creature with one or more counters on it that attacks “if able.” If there’s no other player or planeswalker to attack, the creature simply doesn’t attack.
(wotc • 2021-04-16)
All counters on a creature are considered when determining the value of X, not only +1/+1 counters.
(wotc • 2021-04-16)
Rulings
For each player, choosing a target creature that player controls is optional.
(wotc • 2021-04-16)
Your opponents can choose not to pay to attack with a creature with one or more counters on it that attacks “if able.” If there’s no other player or planeswalker to attack, the creature simply doesn’t attack.
(wotc • 2021-04-16)
All counters on a creature are considered when determining the value of X, not only +1/+1 counters.
(wotc • 2021-04-16)